The principle of an occupant detection device configured to detect whether or not there is a passenger in a vehicle by means of an electrostatic sensor (a capacitance sensor) is explained with reference to FIG. 4. As shown in FIG. 4, an electrostatic sensor 51 that measures capacitance between the electrostatic sensor 51 and a vehicle body 52 is buried in a vehicle seat 53. If a passenger sits on the vehicle seat 53, the capacitance between the electrostatic sensor 51 and the vehicle body 52 is increased, depending on relative dielectric constant of the passenger (human being), compared with the case wherein no passenger sits on the vehicle seat 53. Based on this principle, it is possible to judge whether or not there is a passenger on the vehicle seat 53.
JP-A-2013-186036 has proposed an occupant detection device for a vehicle, which uses the above principle and which achieves accurate detection at low cost even when some liquid is poured (applied) on the vehicle seat or some moisture is absorbed by the vehicle seat. The occupant detection device can achieve the accurate detection by changing the criterion of judgment by taking into account the humidity of environment.
The result of the detection (judgment) is used for another judgment, such as whether an air bag should be activated at a collision accident, or whether a warning about fastening of the seat belt should be issued.
In addition, a seat heater having a heating element may be buried in the vehicle seat 53 in order to warm the hip of a passenger sitting on the vehicle seat 53. The operation of the seat heater may have effects on the measured value of the electrostatic sensor 51. Technique for removing the effects is disclosed in JP-A-2012-035717.